computer-go: [Questions] Null Move

["Mousheng Xu" <moushengxu@xxxxxxxxxxx>]

Some questions about Null Move:

1. What does "-beta + 1" mean in "val = -AlphaBeta(depth - 1 - R, -beta, 
-beta + 1)"? Why set the upperbound to "-beta + 1" when considering a null 
move? Especially, why use "1" instead of other values?

2. I have not seen a Web reference talking about the problem of consecutive 
null moves. In case, say, 2 null moves are used in a row, e.g., Black -> 
White skip to Black -> Black skip back to White, is equivalent to Black -> 
White with depth reduced by R * 2. This is like to evaluate with a shallower 
depth and use it. Is there a danger to use a value with shallower depth?

3. Go is different from Chess in that Go has co-survival but Chess does not. 
When a co-survival is reached, a null move for either side is better than 
any move. In this case, will

...
   val = -AlphaBeta(depth - 1 - R, -beta, -beta + 1);
   if (val >= beta)
       return beta;
...

still work? I guess my problem comes from my poor understanding of "val = 
-AlphaBeta(depth - 1 - R, -beta, -beta + 1)".

Your help is highly appreciated.

Thanks.

-- Mousheng Xu


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